More Wires Need Bigger Boxes

The National Electrical Code limits the number of wires in a box. Here's how to figure the size box you need.

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Box-fill calculations are done to make sure there’s enough room in an electrical box to accommodate safely the wires, splices, cable clamps (usually none in plastic boxes) and outlets or switches that you intend to put in the box.

The volume in cubic inches is usually marked inside nonmetallic boxes. If it’s not, you can measure the inside dimensions of the box and figure it yourself.

Here’s how to figure the size box that complies with the NEC.

Step one

Add up the wires and devices, A K A “conductor equivalents” (wires that start and end in the box — pigtails — aren’t counted)

Each current-carrying wire = 1 All ground wires together = 1 All clamps together = 1 Each receptacle or switch = 2

Step two

Multiply the number of conductor equivalents (total from step one) by their volume factor in cubic inches (listed below)